How to extract a value from a string using regex and a shell?

You can do this with GNU grep's perl mode:

echo "12 BBQ ,45 rofl, 89 lol" | grep -P '\d+ (?=rofl)' -oecho "12 BBQ ,45 rofl, 89 lol" | grep --perl-regexp '\d+ (?=rofl)' --only-matching

-P and --perl-regexp mean Perl-style regular expression.-o and --only-matching mean to output only the matching text.

Yes regex can certainly be used to extract part of a string. Unfortunately different flavours of *nix and different tools use slightly different Regex variants.

This sed command should work on most flavours (Tested on OS/X and Redhat)

echo '12 BBQ ,45 rofl, 89 lol' | sed  's/^.*,\([0-9][0-9]*\).*$/\1/g'

It seems that you are asking multiple things. To answer them:

• Yes, it is ok to extract data from a string using regular expressions, that's what they're there for
• You get errors, which one and what shell tool do you use?

• You can extract the numbers by catching them in capturing parentheses:

  .*(\d+) rofl.*

  and using $1 to get the string out (.* is for "the rest before and after on the same line)

With sed as example, the idea becomes this to replace all strings in a file with only the matching number:

sed -e 's/.*(\d+) rofl.*/$1/g' inputFileName > outputFileName

or:

echo "12 BBQ ,45 rofl, 89 lol" | sed -e 's/.*(\d+) rofl.*/$1/g'